INJURY PREVENTION & PERFORMANCE ENHANCEMENT

R. Barry Dale, PhD, PT, ATC, CSCS, Report Editor

Current Trends in Athletic Shoe Design Marc P. Hilgers, MD, PhD • Florida Orthopaedic Institute; Bernd Mayer, MD, and Markus Walther, MD, PhD • Orthopaedic Hospital Munich-Harlaching, Germany

T

he characteristics of athletic shoes have been described with terms like cushioning, stability, and guidance.1,2 Despite many years of effort to optimize athletic shoe construction, the prevalence of running-related lower extremity injuries has not significantly declined; however, athletic performance has reached new heights.3-5 Criteria for optimal athletic shoe construction Key Points have been proposed, Current trends in athletic shoe design focus but no clear consensus on factors related to individual fit of the has emerged.6-8 Given shoe and force generation that is similar to the unique demands of barefoot walking. various sports, sportspecific shoe designs Cushioning is not as important as once may simultaneously thought in reducing tissue stress. increase performance and decrease injury Medial longitudinal arch support is not as incidence.9-11 The pureffective as once thought in restraining pose of this report is excessive pronation. to provide an overview of current concepts in athletic shoe design, with emphasis on running shoes, so that athletic trainers and therapists (ATs) can assist their patients in selection of an appropriate shoe design.

Basic Construction of an Athletic Shoe An athletic shoe consists of two primary components: (a) the sole and (b) the upper. The upper covers the dorsal, medial, and

lateral aspects of the foot, and the sole provides stability and cushioning at the interface between the ground and the plantar surface of the foot. The shoe upper consists of toe box, heel counter, entry opening, tongue, and quarter. The sole is divided into components that are designated as anterior sole, mid sole/ or brand sole, and outer sole. A foot model, made of wood or plastic, which is used for production of the shoe in the desired size and form, is referred to as a last. Lasts are created to match average measurements from a population. The last used for construction of a shoe produced for North America can differ from the one used for the Asian market. Lasts are designated as straight, half-bend, or full bend. The bend form is generally considered to produce a shoe that is more comfortable because most feet are slightly inverted. Straight-last shoes offer more stability, which can be beneficial for an athlete who has flexible feet.

Cushioning The relationship between impact force at heel strike and musculoskeletal injury was originally based on animal models and theoretical calculations. To date, this relationship has not been established in human models.2,12-14 New biomechanical techniques allow for calculation of stress within biologic structures. Stress on cartilage, ligaments, bones, and tendons of the lower extremity © 2009 Human Kinetics - ATT 14(6), pp. 4-8

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are independent of, or only minimally affected by, the cushioning property of the shoe sole.15-17 Stress is a determined by joint geometry, muscle tension, and external forces.18 The contribution of soft tissue vibration related to heel strike is also being investigated. Soft tissue vibration creates an unpleasant sensation that is reduced by muscle activation.19,20 Changes in soft tissue vibration and muscle activation could explain the differing sensations experienced when running on soft versus hard ground or running in cushioned versus firm shoes. Electromyography (EMG) changes support this theorized relationship.21,22 Research evidence suggests that the impact forces generated during “normal” athletic activity have no significant role in the development of injuries. Shoe cushioning does appear to have a significant influence on comfort, muscle activation, performance, and exhaustion, however.8,18 How much cushioning is sensible? Throughout most of history, humans walked barefoot on natural ground. Anthropologists estimate that early humans averaged a daily walking distance of up to 40 km,23 and the ability to run fast may have provided a selection advantage. Current shoe cushioning rationale reflects an effort to diminish the differences between natural terrain and artificial surfaces. Excessive cushioning appears to be as problematic as insufficient cushioning.24,25

30 km per week of running over a 6-months period demonstrated that axial orientation of foot and ankle joints did not predict running-related injuries.40 Measurements of the bone displacements during running have failed to demonstrate any consistent relationship with medial support.41.42 Inverse dynamic analysis, which uses kinematic and kinetic measurements to calculate stress levels within biologic structures, has demonstrated a relationship between foot pronation and high levels of tissue stress. Analysis of simultaneous multisegmental displacements appears to be important for identification of the causes or overuse injury syndromes in the lower extremity. Medial tibial stress syndrome, ilioltibial band syndrome, and patellofemoral pain syndrome have been related to forces created by a combination of excessive foot pronation, leg internal rotation, and leg abduction.43 Maximum foot pronation and pronation velocity appear to play a greater role in development of overuse injury than the magnitude of the ground reaction force that is generated at heel impact.30 The peak vertical ground reaction force generated by barefoot walking be can be as much as two times greater than that which is generated while wearing a cushioned athletic shoe44 (Figure 1).

Motion Control

The term “comfort” is associated with both the fit of a shoe and the extent to which the shoe supports the “natural motion” of the foot.45 Athletes view the fit of the shoe as one of its most important characteristics.46,47 The mechanical qualities of a shoe’s design can be objectively quantified, but the shoe’s fit involves interaction between shoe’s configuration and

Excessive foot pronation and associated internal leg rotation has been linked to development of overuse syndromes of the knee (e.g., patellofemoral pain syndrome, ilioltibial band syndrome).26-30 Medial tibial stress syndrome, Achilles’ tendinopathy, plantar fasciitis, and stress fractures have also been related to excessive foot pronation and internal leg rotation.13,31 As a result of this relationship, restraint of excessive motion has been viewed as a paramount requirement of running shoes and orthotics. Despite widespread belief that support of the medial longitudinal arch is important for correction of over-pronation, the effects of orthotics on foot and leg motion have been found to be minor and inconsistent.32-37 In addition, integration of stiff medial components into the shoe sole (“second density”) to provide a increased mechanical resistance to foot pronation was unsuccessful in significantly restraining over-pronation of the foot.38,39 The findings of a prospective study of 131 runners who averaged

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Fit

Figure 1  Cushioned athletic shoe.

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the individual athlete’s foot, which extremely difficult to measure. Each of these three factors influences the movement of the foot within the shoe. Runners can subjectively judge the fit of shoe very well.48 There are significant differences in foot shape between male and female runners.22 Research has demonstrated that a runner’s preferred running shoe is always positioned well in relation to the foot structure.45 However, fit in the heel area could not be conclusively correlated with a reduction of pronation velocity.39,49

Athletic Shoe Industry Response to Recent Findings Based on recent findings, the two biggest trends in athletic shoe design are individualization of fit and force generation that is similar to barefoot walking on a natural ground.50 Various companies make application of these concepts in different ways. An example of the individualization of fit is the availability of various widths from New Balance™. Their four top-selling models are available in 4 different widths for each shoe size. Width individualization is limited to the forefoot; the midfoot and hindfoot are identical in all 4 models. Other companies offer as many as 3 widths for certain models. Due to costs associated with longer shelf time, this fit individualization concept has not been readily accepted by retail stores that sell athletic shoes. Frequently, mid-range widths are the only ones that are available in stores.51 The Biomorphic fit system from Asics™ provides soft elastic areas in the upper of the forefoot. This design feature promises that no pressure points will be created, even with mild hallux valgus or a tailor bunion. Product lines are available that provide adjustments for major differences between the foot structure of male and female athletes, and those of different ethnic backgrounds.22,31,47 Nike™ was the one of the first companies to offer “female” running shoes, which can be found in most major athletic shoe retail stores. In addition to the differences in foot structure, the lower average body weight of female athletes is considered in the degree of sole flexibility. Adidas™ has introduced “mi Adidas,™” individualized shoes, which accommodate a wide variety of different foot types. Foot measurements are utilized to optimize shoe fit for running shoes, soccer shoes, tennis shoes, basketball shoes, and handball shoes.

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Differing cushioning needs can be accommodated by shoes having different softness levels. This is made possible through the use of PU-foam and EVA-foam, honeycomb structure (Figure 2), or gel. A new and innovative concept is an electronic cushioning adjustment that involves a servo motor, which alters the pretension level of a cushioning module on the heel. The first shoe of this type (marketed as the Adidas 1™”) was too heavy for acceptance by athletes. Its latest version (the “SMART RIDE”) offers greater flexibility and is much lighter in comparison to the original version that was introduced in 2005. Reduction of pronation moment arm length in the hindfoot can be achieved by specifically designed zones in the sole that readily deform upon heel impact. The “crash pad™” by Nike™ is an example of a shoe design feature that utilizes this concept (Figure 3). Reebok™ created holes in the sole material in the heel area, thereby allowing a few millimeters of movement that reduces pronation-inducing torque during the first few milliseconds of ground contact. A complete detachment of the hindfoot section is created by the Adidas™ ForMotion™ design. The outer sole slides on a half-cup configuration, thereby reducing for the conversion of vertical ground reaction force into rotational torque. The reduction of pronation torque, which is associated with multiple running injuries, is the primary goal (Figure 4). The most innovative step toward replication of barefoot walking has been developed by Nike™. The “Nike Free™” shoe is characterized by the complete absence of any stabilizing elements (Figure 5). Its cushioning is designed to mimic forces associated with barefoot contact against natural ground. This shoe requires considerable muscle activation for stabilization of the foot. Many athletes develop muscle soreness after wearing the shoe for the first time. Recently, this shoe has been marketed as a training tool, rather than a running shoe.

Conclusion The design of athletic shoes has historically reflected emphasis on cushioning, stabilization, and guidance. These concepts have progressively been replaced by emphasis on the individualized considerations, such as sport-specific requirements and the individual athlete’s foot structure. Recently introduced athletic shoe designs incorporate mechanisms to reduce pronation

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Figure 2  PU-foam and EVA-foam, honeycomb structure. Figure 3  “Crash pad™” by Nike™.

Figure 4  Adidas™ ForMotion™ design.

torque, and cushioning is limited to a sport-specific requirement. Recent athletic shoe design concepts are largely based on the idea that the musculoskeletal system of humans has adapted to barefoot walking on a natural surface for thousands of years.52 

References 1. Segesser B, Nigg BM. Orthopadische und biomechanische Konzepte im Sportschuhbau. Sportverletz Sportschaden. 1993;7:150-62.

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Figure 5  The “Nike Free™” shoe.

2. Nigg BM, Segesser B. Der Laufschuh - Ein Mittel zur Prävention von Laufbeschwerden. Z Orthop. 1986;124:765-71. 3. Taunton JE, Ryan MB, Clement DB et al. A retrospective case-control analysis of 2002 running injuries. Br J Sports Med. 2002;36:95101. 4. Taunton JE, Ryan MB, Clement DB et al. A prospective study of running injuries: the Vancouver Sun Run “In Training” clinics. Br J Sports Med. 2003;37:239-44. 5. van Mechelen W. Running injuries. A review of the epidemiological literature. Sports Med. 1992;14:320-35. 6. Walther M. Neue Trends im Sportschuhbau - Eine Literaturübersicht. Fuss Sprunggelenk. 2004;2:167-75.

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7. Nigg BM. The role of impact forces and foot pronation: a new paradigm. Clin J Sport Med. 2001;11:2-9. 8. Brüggemann GP. Neue Aspekte zum Laufschuh. Sportorthopädie Sporttraumatologie. 2003;19:93-5. 9. Walther M, Reuter I, Leonhard T et al. Injuries and response to overload stress in running as a sport. Orthopade. 2005;34:399-404. 10. Ceroni D, De R, De CG et al. The importance of proper shoe gear and safety stirrups in the prevention of equestrian foot injuries. J Foot Ankle Surg. 2007;46:32-9. 11. Lake MJ. Determining the protective function of sports footwear. Ergonomics. 2000;43:1610-21. 12. Radin EL, Parker HG, Plugh JW et al. Response of joint impact loading - relationship between trabecular microfractures and cartilage degeneration. J Biomech. 1993;6:51-7. 13. James SL, Bates BT, Osternig LR. Injuries to runners. Am J Sports Med. 1978;6:40-50. 14. Grau S, Horstmann T. Dämpfung in der Laufschuhforschung. Sportorthopädie Sporttraumatologie. 2003;19:85-90. 15. Cole GK, Nigg BM, Fick GH et al. Internal loading of the foot and ankle during impact in running. J Appl Biom. 2000;11:25-46. 16. Morlock M. A generalized three-dimensional six-segment model of the ankle and the foot. Alberta, Canada: University of Calgary; 1990. 17. Kong PW, Candelaria NG, Smith D. Running in new and worn shoes: a comparison of three types of cushioning footwear. Br J Sports Med. 2008. 18. Nigg BM, Wakeling JM. Impact forces and muscle tuning: a new paradigm. Exerc Sport Sci Rev. 2001;29:37-41. 19. Nigg BM, Nurse MA, Stefanyshyn DJ. Shoe inserts and orthotics for sport and physical activities. Med Sci Sports Exerc. 1999;31:S421-S428. 20. Nurse MA, Nigg BM. Quantifying a relationship between tactile and vibration sensitivity of the human foot with plantar pressure distributions during gait. Clin Biomech. (Bristol , Avon ) 1999;14:667-72. 21. Nigg BM. Impact forces in running. Curr Opin Orthop. 1997;8:43-7. 22. O’Connor K, Bragdon G, Baumhauer JF. Sexual dimorphism of the foot and ankle. Orthop Clin North Am. 2006;37:569-74. 23. Lee RB, DeVore I. Man the Hunter. Chicago: Aldine; 1968. 24. Robbins SE, Gouw GJ. Athletic footwear: unsafe due to perceptual illusions. Med Sci Sports Exerc. 1991;23:217-24. 25. Robbins SE, Hanna AM. Running-related injury prevention through barefoot adaptations. Med Sci Sports Exerc. 1987;19:148-156. 26. Hintermann B, Nigg BM. Bewegungsübertragung zwischen Fuß und Unterschenkel in vitro. Sportverletz Sportschaden. 1994;8:60-6. 27. Messier SP, Edwards DG, Martin DF et al. Etiology of iliotibial band friction syndrome in distance runners. Med Sci Sports Exerc. 1995;27:95160. 28. Hintermann B, Nigg BM. Pronation in runners. Implications for injuries. Sports Med. 1998;26:169-76. 29. Busseuil C, Freychat P, Guedj EB et al. Rearfoot-forefoot orientation and traumatic risk for runners. Foot Ankle Int. 1998;19:32-7. 30. Grau S, Maiwald C, Krauss I et al. What are causes and treatment strategies for patellar-tendinopathy in female runners? J Biomech. 2008;41:2042-2046. 31. Cavanagh PR. The running shoe book. Mountain View, CA: Anderson World Inc.; 1980. 32. Nigg BM, Morlock M. The influence of lateral heel flare of running shoes on pronation and impact forces. Med Sci Sports Exerc. 1987;19:294302.

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33. Smith LS, Clarke TE, Hamill CL et al. The effects of soft and semi-rigid orthoses upon rearfoot movement in running. J Am Podiatr Med Assoc. 1986;76:227-33. 34. Eng JJ, Pierrynowski MR. The effect of soft foot orthotics on threedimensional lower-limb kinematics during walking and running. Phys Ther. 1994;74:836-44. 35. McCulloch MU, Brunt D, Vander LD. The effect of foot orthotics and gait velocity on lower limb kinematics and temporal events of stance. J Orthop Sports Phys Ther. 1993;17:2-10. 36. Nawoczenski DA, Cook TM, Saltzman CL. The effect of foot orthotics on three-dimensional kinematics of the leg and rearfoot during running. J Orthop Sports Phys Ther. 1995;21:317-27. 37. Davis IS, Zifchock RA, Deleo AT. A comparison of rearfoot motion control and comfort between custom and semicustom foot orthotic devices. J Am Podiatr Med Assoc. 2008;98:394-403. 38. Grau S, Baur H, Horstmann T. Pronation in der Sportschuhforschung. Dtsch Z Sportme. 2003;54:17-24. 39. Stacoff A, Reinschmidt C, Nigg BM et al. Effects of shoe sole construction on skeletal motion during running. Med Sci Sports Exerc. 2001;33:311-9. 40. Bahlsen HA. The Ethiology of Running Injuries: A Longitudinal Prospective Study. Alberta, Canada: University of Calgary; 1989. 41. Nigg BM, Khan A, Fisher V et al. Effect of shoe insert construction on foot and leg movement. Med Sci Sports Exerc. 1998;30:550-5. 42. Stacoff A, Reinschmidt C, Nigg BM et al. Effects of foot orthoses on skeletal motion during running. Clin Biomech (Bristol , Avon). 2000;15:54-64. 43. Stefanyshyn DJ, Stergiou P, Lun VM et al. Knee angular impulse as a predictor of patellofemoral pain in runners. Am J Sports Med. 2006;34:1844-51. 44. Stacoff A, Denoth J, Kaelin VX et al. Running injuries and shoe construction. Int J Sports Biomech. 1988;3:342-57. 45. Walther M. Passform und Bewegungsmuster beim Sportschuh. Orthopädieschuhtechnik. 2003;11:45-48. 46. Kleindienst FI, Krabbe B, Walther M et al. Grading of the functional sport shoe parameter “cushioning” and “forefoot flexibility” on running shoes. Sportverletz Sportschaden. 2006;20:19-24. 47. Kleindienst F. Gradierung funktioneller Sportschuparameter am Laufschuh - in Bezug auf eine anthropometrische Differenzierung, geschlechtsspezifische Gradierung und geographische Gradierung. Köln: Disseration Deutsche Sporthochschule; 2003. 48. Milani TL, Kimmeskamp S, Hennig EM. Zusammenhang von biomechanischen Parametern und subjektiver Belastungswahrnehmung in Laufschuhen. Dtsch Z Sportme. 1997;48:139-44. 49. Stacoff A, Nigg BM, Reinschmidt C et al. Tibiocalcaneal kinematics of barefoot versus shod running. J Biomech. 2000;33:1387-95. 50. Walther M, Mayer B. Aktuelle Trends in der Sportschuhentwicklung. Dtsch Z Sportme. 2008;59:12-6. 51. Walther M, Herold D, Sinderhauf A et al. Children sport shoes--a systematic review of current literature. Foot Ankle Surg. 2008;14:180-9. 52. Squadrone R, Gallozzi C. Biomechanical and physiological comparison of barefoot and two shod conditions in experienced barefoot runners. J Sports Med Phys Fitness. 2009;49:6-13.

Marc Hilgers is with the Florida Orthopaedic Institute in Tampa, FL. Bernd Mayer and Markus Walther are with the Orthopaedic Hospital Munich–Harlaching in Munich Germany.

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